1. Field of the Invention
The present invention relates to a focus detection apparatus of a camera, which detects the focusing condition of a photographic lens of the camera with respect to a predetermined focal plane.
2. Description of the Prior Art
FIG. 1 is an outline perspective view of a prior art focus detection apparatus operating with a phase difference detection principle. In FIG. 1, L.sub.0 represents a condenser lens and L.sub.1 and L.sub.2 show a pair of re-imaging lenses arranged substantially symmetrical to the optical axis l.sub.0 of a photographic lens, not shown, of the camera. The optical axis l.sub.0 may be referred to as "main optical axis" in the following description.
The overall image formed by the photographing or objective lens is reformed through the condenser lens L.sub.0 and re-imaging lenses L.sub.1 ; L.sub.2 into first and second images. In this case, the distance between these first and second images will be subjected to a variation, depending upon the focusing condition of the photographing lens.
Thus, when first and second rows I and II of light reception elements of a line sensor P.sub.0 are arranged at a conjugate position of the predetermined focal plane FP of the photographing lens or at a close position thereof with the condenser lens L.sub.0 and the refocusing lenses L.sub.1 ; L.sub.2 as shown, the relative position of said first and second images represents the focusing condition of the photographic lens. By sensing the thus caused relative positional variation with the use of the first and second light reception element rows I; II, the focusing condition of the photographing lens can be detected.
In the case of such a focus detecting apparatus having the optical system set forth above, if the center-to-center line of re-imaging lens pair L.sub.1 ; L.sub.2 should incline, even the slightest, from the common and extending line of the first and second light reception element rows I, II, the now formed first and second images will be subjected disadvantageously to a shifting widthwise of first and second light reception element rows I; II, whereby a kind of squint cast phenomenon is created. If this phenomenon should occur to take place, the focus detection accuracy would be considerably reduced.
Now assuming that the center-to-center distance between the both re-imaging lenses L.sub.1 ; L.sub.2 is measured to 1, and the center O' of lens L.sub.2 is positioned at a higher level by a distance .DELTA..sub.y than the center O of lens L.sub.1 at a plane which includes the main optical axis l.sub.0 of the photographing lens and which extends perpendicular to the extending direction of the first and second light reception element rows I; II, or in other words that the line segment O-O' is inclined by an angle .theta.=tan.sup.-1 (.DELTA..sub.y /l) relative to the element rows I; II. It is further assumed that I' and II' represent reversedly projected theoretical images of said element rows I and II on the predetermined focal plane FP through the reimaging lenses L.sub.1 ; L.sub.2 and condenser lens L.sub.0.
Now referring to FIG. 2, showing a considerably enlarged views from the foregoing, an image shifting by k .DELTA..sub.y (k being a certain constant which is a reciprocal of magnification .beta. of the re-imaging lens) is clearly seen to have occurred between the both images I'; II'. Reversely, it will be seen that at the predetermined focal plane FP, the regional images I'; II' will be reproduced on the first and second light reception element rows I; II, respectively, and with the image shifting of k.DELTA..sub.y. In this case, it should thus be noted that a distinct and clear error should occur in the focus detection operation, only with an exception of the case of a vertical line image formed on the predetermined focal plane FP.
As for the counter measure to prevent the squint cast phenomenon in the case of single focus detection area employed, reliance may be had on the proposal shown and described in Japanese Patent Publication (unexamined) No. Sho-60-31109. Another solution is to execute precisely adjustment parallelism between the segmental line O-O' connecting the centers of re-imaging lens pair L.sub.1 ; L.sub.2 and the extending direction of line sensor P.sub.0 to a high precision degree of plus/minus 5 minutes or so.
However, in the case of a camera, provided with a plurality of focus-detection assemblies each including a condenser lens L.sub.0 and a pair of re-imaging lenses L.sub.1 and L.sub.2, it would be insufficient to rely upon the foregoing mode of adjustment, in order to provide positioning as required for all the included re-imaging lens pairs as well as all the included line sensors, because of the fact that these line sensors or re-imaging lens element pairs must be arranged separately and independently movable. Taking the line sensors as an example, all these line sensors can not be formed on one and the same tip, thereby inviting a considerable costup of the manufacture and a considerable increase of AF sensor module to be employed. Further, it is necessary to execute a plurality of adjustments during the manufacturing step, inviting a substantially reduced productivity.
Further, even if the plurality of re-imaging lens pairs could have been produced into one piece such as a solid molded piece, a certain "squint cast" phenomenon could disadvantageously appear at least in one or other of the focus detection areas on account of a smaller degree of freedom in adjustability, when making necessary positional adjustments of the chip formed with line sensors as well as re-imaging lens pairs, and indeed, by reason of regularity unavoidable errors in the lens machining preciseness can occur.